Mark ABSL_DEPRECATED() as [[deprecated]]

PiperOrigin-RevId: 985436747
Change-Id: I845e5cf76a90ad029b46c1e364afe3b01426e0eb

absl/base/attributes.h

diff --git a/absl/base/attributes.h b/absl/base/attributes.h
index da12f1f..596fdbb 100644
--- a/absl/base/attributes.h
+++ b/absl/base/attributes.h
@@ -646,8 +646,7 @@
 // declarations. The macro argument is used as a custom diagnostic message (e.g.
 // suggestion of a better alternative).
 //
-// For code or headers that are assured to only build with C++14 and up, prefer
-// just using the standard `[[deprecated("message")]]` directly over this macro.
+// Deprecated: Use the standard `[[deprecated("message")]]` macro instead.
 //
 // Examples:
 //
@@ -668,7 +667,25 @@
 // turns this warning off by default, instead relying on clang-tidy to report
 // new uses of deprecated code.
 #if ABSL_HAVE_ATTRIBUTE(deprecated)
+#if defined(__cplusplus) && defined(__clang__) && \
+    ABSL_HAVE_ATTRIBUTE(diagnose_if) && !defined(SWIG)
+struct [[deprecated(
+    "Use [[deprecated(...)]] instead.")]] _absl_deprecated_macro;
+// Trick: We use __attribute__((diagnose_if(...))) to refer to our own
+// deprecated symbol, which then causes a deprecation message to be emitted when
+// the macro is used. Since diagnose_if() isn't valid on every declaration, we
+// also suppress the warning regarding that.
+#define ABSL_DEPRECATED(message)                                             \
+  _Pragma("clang diagnostic push") /*                                     */ \
+      _Pragma("clang diagnostic ignored \"-Wignored-attributes\"")           \
+          __attribute__((diagnose_if(                                        \
+              sizeof(_absl_deprecated_macro*) == 0, "",                      \
+              "warning"))) /*                                             */ \
+          _Pragma("clang diagnostic pop") /*                              */ \
+      __attribute__((deprecated(message)))
+#else
 #define ABSL_DEPRECATED(message) __attribute__((deprecated(message)))
+#endif
 #else
 #define ABSL_DEPRECATED(message)
 #endif
@@ -678,7 +695,7 @@
 // deprecated code can be surrounded with these directives to achieve that
 // result.
 //
-// class ABSL_DEPRECATED("Use Bar instead") Foo;
+// class [[deprecated("Use Bar instead")]] Foo;
 //
 // ABSL_INTERNAL_DISABLE_DEPRECATED_DECLARATION_WARNING
 // Baz ComputeBazFromFoo(Foo f);